Electrostatic precipitators and other dust collectors, operating with housings which receive explosive or otherwise expandable mixtures, are customarily provided with pressure-relief openings through which a sudden increase of pressure can be vented to prevent a shock wave or like contained expansion or explosion of gases from destroying the dust collector or internal fixtures thereof.
Electrostatic precipitators have been used heretofore where the composition of the gas or the nature of the dust to be collected resulted in an expectation of detonation or explosion even under normal operating conditions. Typical of this first class of dust collectors or electrostatic precipitators are those which follow steel-making converters to effect at least partial cleaning of the gases drawn therefrom before these gases are released into the atmosphere. Explosive components of such gases include carbon monoxide and particulates which undergo violent or spontaneous chemical reactions producing more moles of reaction product than moles of reactant.
A second class of electrostatic precipitators are those which are not operated under the expectation of detonation or explosion in normal usage, but wherein detonation or explosion cannot be precluded when troubles arise in the operation of the installation. Such systems include gas purifiers following heat-exchange furnaces or rotary kilns in the cement industry.
A third class of dust collector or heat exchanger is that in which detonation or explosion cannot occur at all. Typical of these dust collectors or electrostatic precipitators are those which follow grinders, millers and steam boilers.
With dust-collecting installations of the latter type, the housing can be designed simply to withstand the pressure generated in normal use and ranging from slightly above atmospheric pressure (0 bar) to -2000 millibar (mb).
In the two fields of application originally mentioned, however, the housing must be designed to withstand not only the normal operating pressure but explosion pressures, detonation waves and the like which may be as high as 12 bars.
For economic reasons, it is neither practical nor possible to design structures so massive as to be capable of withstanding these pressures.
As a result, it is a practical necessity to provide pressure-relief openings with closure members, e.g. explosion-responsive hinged doors, to limit the pressure rise in the case of an explosion or detonation.
When such pressure-relief openings are provided, naturally, the pressure buildup within the dust collector is vented as the doors are opened. The size of the pressure-relief openings and the threshold pressure at which the doors respond are so selected that the housing can have sufficient strength for normal operations with the vents closed, but nevertheless the cost of the unit can be minimized. A typical threshold in the first case, i.e. when detonation or explosion may be expected with normal operation is generally around 1.5 to 2 bar.
In other words, at pressures of this level, the housing cannot stand any significant permanent deformation upon the development of an explosive force.
Naturally, certain relationships between relief-apertures cross section and threshold pressure must be observable Because the detonations and explosions occur relatively frequently, the pressure relief means which are employed must automatically reestablish a gas-tight seal after an explosion. Under these conditions, the dust collector or precipitator may continue in operation after many detonations or explosions without the need for repair. Consequently, while the initial cost for a pressure vessel and resealing pressure-relief openings may be relatively high, the number of explosive incidents precludes underdesigning in any system which would require frequent maintenance or replacement of parts, in the second field of application mentioned above, i.e. those in which detonations or explosions cannot occur under normal operating conditions but nevertheless may occur infrequently in the event of operating problems, a different approach may be taken.
In the latter case it may be more economical to design the housing so that it can respond to pressure surges, i.e. to be resistant to a pressure surge. This means that infrequent permanent deformation of housing walls can result from explosions while subsequent repair or replacement is tolerable. An advantage of this system, of course, is that the housing is usually designed so as not to burst with the usual explosive forces which may be expected upon a failure of the operating system. The design can be based upon the yield point of ferritic steels or the 1% offset point of austenitic steels as measures of the permissible stress to which wall members of these steels may be subjected. No margin of safety need be given since replacement of damage from an explosive incident is taken into consideration.
In systems of the latter case it has also been found to be advantageous to minimize the replacement cost and frequency of repair, to provide pressure-relief openings. It is not unusual, with these systems, to provide a pressure-relief opening which vents or responds at a threshold pressure of 0.25 bar and, because of the low threshold relief pressure, to provide a correspondingly larger flow cross section for the pressure relief means.
For systems in which detonations or explosions are not expected during normal operation but cannot be precluded in the event of operational problems, e.g. in exhaust gas purifying installations or dust collectors downstream of heat exchange furnaces or rotary kilns in the cement industry, it has been proposed (see German Pat. No. 1,297,082) to provide a dust collecting electrostatic precipitator whose housing top is constituted as an explosion-responsive flap.
The top of the dust collector is resiliently clamped and held in a gas-type manner against roof girders and side walls of the dust collector and is subdivided into a number of strips which are sealed, in turn, to obtuse-angle strips. In the event of explosion, these angle strips are bent more sharply upwardly so that the several segments approach one another and the top of the dust collector is effectively reduced in length and is pulled out of the means whereby its edges are gripped, thereby enabling the entire top to be raised in the relief of pressure within the housing.
While this system is effective for limited relief, it does not fulfill all of the requirements since an explosion in the dust collector results in a rapid pressure rise and high-speed shock waves whose destructive effect can only be limited if the pressure-relief means responds rapidly and forms a pressure relief opening of sufficient area.
With the system just described, the mass and inertia of the parts which had been deformed to expose the pressure relief cross section were such that significant delay was created and hence the system was unsatisfactory because damage to the housing walls could not be avoided. An additional defect in earlier systems of this type was that the clamping systems used for securing the top of the housing to the walls did not permit a well-defined release of the pressure and hence the overall system did not have a predetermined response threshold pressure. In other words, while it is a requirement that the housing be sealed, the sealing means used did not permit the pressure-relief system, as a whole, to have a predetermined threshold value at which the internal pressure was reliably relieved at a rate sufficient to preclude damage to the balance of the collector. In fact, the clamping characteristics changed as a function of weather and rendered the system unreliable.
Finally, in this connection, it was found that the pressure wave tore away all covering for the collector so that the latter was not even left with a rain-shielding roof or the like.